1. Field of the Invention
This invention relates to a method of reproducing an image which is taken with an irradiation field limited as a visible image on a light emission display means such as a CRT.
2. Description of the Related Art
There has been known a medical image display system in which, for instance, a stimulable phosphor sheet is exposed to a radiation passing through an object to have a radiation image of the object stored thereon and then exposed to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy, the light emitted from the stimulable phosphor sheet is photoelectrically detected and converted to an electric image signal, and the radiation image of the object is reproduced as a visible image on a light emission display means such as a CRT on the basis of the electric image signal.
In such a system, the image signal is subjected to image processing so that the radiation image is reproduced according to gradation properties where the brightness level of a picture element becomes higher as the level of the image signal component for the picture element becomes lower and the radiation image is reproduced as a visible image by use of the processed image signal.
When recording a radiation image, it is sometimes desired that portions of the object not related to diagnosis or the like be prevented from being exposed to the radiation. Further when the object portions not related to diagnosis or the like are exposed to the radiation, the radiation is scattered by such object portions to the portion related to diagnosis or the like and the image quality is deteriorated. Accordingly a radiation image is sometimes recorded using an irradiation field stop so that only a necessary portion of the object is exposed to the radiation. Conventionally also a radiation image taken using such an irradiation field stop is reproduced according to gradation properties where the brightness level of a picture element becomes higher as the level of the image signal component for the picture element becomes lower as a radiation image taken without using an irradiation field stop.
In the case of a radiation image taken using an irradiation field stop, the image signal components obtained from the area of the stimulable phosphor sheet outside the irradiation field are very low in level since the area is not exposed to the radiation and the amount of light emitted from the area upon stimulation is very small. As a result, the area outside the irradiation field is reproduced at a high brightness on the CRT or the like. To the contrast, the image signal components obtained from the area of the stimulable phosphor sheet inside the irradiation field are high in level since the area inside the irradiation field is exposed to the radiation and the amount of light emitted from the area upon stimulation is in proportional to the radiation energy. As a result, the area inside the irradiation field is reproduced on the CRT or the like at a brightness lower than the area outside the irradiation field.
The image reproduction described above is for reproducing a radiation image, which is stored on a stimulable phosphor sheet and in which the image signal level of the area outside the irradiation field is lower than that of the area inside the irradiation field, on a light emission display means such as a CRT according to gradation properties where the brightness level of a picture element becomes higher as the level of the image signal component for the picture element becomes lower. There also has been known an image reproduction where an X-ray image of an object is recorded on an X-ray film, the X-ray film is developed, an image signal is obtained by scanning the developed X-ray film and reading out transmitted light or reflected light (digitization of an X-ray film), and the X-ray image is reproduced as a visible image on a light emission display means such as a CRT on the basis of the image signal.
In this case, the image signal level of the area outside the irradiation field is higher than that of the area inside the irradiation field and the X-ray image is reproduced on the light emission display means according to gradation properties where the brightness level of a picture element becomes higher as the level of the image signal component for the picture element becomes higher. Also in this case, the area inside the irradiation field is reproduced on the CRT or the like at a brightness lower than the area outside the irradiation field.
However reproducing an image on a light emission display system such as a CRT involves a difficulty that when an image area at a high brightness level exists adjacent to an image area at a low brightness level, the contrast of the image in the low brightness level area is deteriorated due to scattered light from the high brightness level area.
For example, in the case of a CRT, phosphors emit light and glass plate exists on the front face of the CRT. There are scattered light from the light emitted from the phosphors and scattered light scattered at the glass plate. When there is a high brightness level area adjacent to a low brightness level area, the brightness level in the low brightness level area is increased by scattered light from the phosphors in the high brightness level area and scattered light at the glass plate, which results in deterioration in contrast.
Thus there has been a problem in the conventional image reproducing method that when the image to be reproduced is an image taken using an irradiation field stop, the contrast of the image inside the irradiation field (low brightness lever area) is deteriorated by scattered light from the image outside the irradiation field (high brightness level area), which is undesirable in viewing for diagnostic purposes.